DISTRIBUTION OF NATURALLY OCCURRING RADIONUCLIDES (K, TH AND U) IN WEATHERED ROCKS OF VARIOUS LITHOLOGICAL TYPES FROM THE URANIUM BEARING REGION OF FORNOS DE ALGODRES, PORTUGAL

Abstract

The distribution of K, Th and U in various types of weathered rocks (granite, schist, dolerite and aplite), belonging to the Beiras uraniferous province in the Fornos de Algodres area of Northern Portugal, was studied. Since the concentration of naturally occurring radionuclides in rocks depends much on the geological context, levels of ionizing radiation and radon emission may vary as a function of geological origins and processes. Our study aims to contribute to the knowledge of the background concentrations of these elements from rocks in a region that is generally enriched in uranium, and consequently with higher potential risk. These are used to estimate dose rates to which people might be exposed at the land surface, as well as dose rates within each context, relevant to luminescence dating, and identify sources of enhanced radon emission.

The concentrations of natural radionuclides were obtained using two methods for comparison, the field gamma spectrometry (FGS) and instrumental neutron activation analysis (INAA), as their combination may help detect spatial variations and disequilibrium in the U series. The mineralogical study of rock types was performed by X-ray diffraction of powder samples.

INAA and FGS results were commonly similar. The K, Th and U contents in the studied area are variable, but usually are higher than average upper continental crust values. A granitic weathering profile was encountered in which significant leaching of K had occurred, producing a ca. 30% difference in dose rate. The greatest U contents were observed in a weathered aplite dyke intersecting the granite; radon escape of up to 590 Bq kg-1 was inferred, producing a 300% difference between dose rates based on INAA or FGS data. However, the enrichment in U is not restricted to aplite; its heterogeneous distribution may cause localized enrichment, which was detected in the other lithologies: notably, soils on moderately radioactive bedrock were enriched in Th and U by colluvial movement.